One of the first soccer games I attended in Brazil was a battle between Minas Gerais interstate rivals Cruzeiro and Atlético Mineiro. The crowd of more than 50,000 was evenly split between fans of each team, with each side separated from one another in the stadium.
During intermission, I wandered around the stadium, coming to the area where police had cordoned off the sides. On one side were fans of Cruzeiro, the other side fans of Atlético, both sides taunting one another. Then, the taunting got too intense. And the police tear gassed the lot of us.